Process for obtaining a synthetic organic aromatic heterocyclic rod fiber of film with high tensile strength and/or modulus

ABSTRACT

The invention pertains to a process for obtaining a synthetic organic aromatic heterocyclic rod fiber or film with high tensile strength and/or modulus comprising spinning a synthetic organic polymer to a aromatic heterocyclic rod fiber or obtaining the synthetic organic polymer as an aromatic heterocyclic rod film, followed by loading the fiber or film in the presence of a processing aid, at a temperature below the boiling point of the processing aid and above 50° C., at a tension of 10 to 95% of the fiber or film breaking strength, followed by removing the processing aid and/or performing a heating step at a tension of 10 to 95% of the fiber or film breaking strength.

BACKGROUND

The invention pertains to a fiber or film and a process for obtaining asynthetic aromatic heterocyclic rod organic fiber or film with hightensile strength and/or modulus.

For many high-tech applications it is important to use fibers and filmswith high tensile strength and/or modulus. These so-calledhigh-performance fibers or films may be organic-based (e.g. para-aramidfibers and films or carbon fibers) or inorganic (e.g. E-glass fibers,silicon carbide fibers). These high performance fibers or films are usedin numerous specialty products for automotive, aerospace and ballisticapplications, reinforcement of constructions, offshore exploration,protective apparel, sports equipment, and thermal insulation. Each typeof high-performance fiber or film excels in certain niche applications.

A special type of high performance fibers or films is high-modulushigh-tenacity fibers or films. Organic members of this group containcovalent (one-dimensional) chains that are held together byintermolecular interactions. Typical examples are ultra-high-molecularweight poly ethylene (UHMW PE) like Dyneema® and Spectra®, para-aramidslike Kevlar®, Technora® and Twaron®, aromatic homocyclic polyesters likeVectran®, and aromatic heterocyclic rods like poly(p-phenylenebenzobisoxazole (“PBO”) (Zylon®) and poly(pyridobisimidazole (“PIPD”)(M5) based on pyridobisimadazole.

PBO combines high modulus and tenacity with good thermal properties andflexibility, making it suitable in ballistics, flame resistant work wearfor fire fighters and heat resistant felts. Application in structuralcomposites, however, is limited by its low compressive strength. The newfiber or film M5 is a PBO-like fiber or film with significantly improvedcompression behavior.

Up to now it was believed that the above fibers or films span animpressive range in tensile properties, some of them even within onefiber or film type. Nevertheless, when the tensile strengths could beincreased further, a substantial improvement could be obtained, evenmaking available new applications that are not yet possible with theexisting high-performance fibers or films. For PIPD the conventionaltechnique of spinning, air gap drawing, and heat treatment has beendescribed in EP 0,696,297, which technique is considered the closestprior art.

According to the existing methods, the orientation and the modulus offibers and films is improved by a heat treatment under tension. So, forinstance, an oven is used for fibers, which consists of a (quartz) tube.Into the tube, slightly above the bottom, a flow of nitrogen isintroduced. Its flow rate can be controlled and it can be heated. Thenitrogen flow is used to heat the fiber and in addition serves as aninert atmosphere. The fiber is suspended from an upper-clamp, throughthe oven. To its lower end, a weight is connected which applies thetension during the treatment. Both, oven and upper-clamp are mounted toa solid frame. The second clamp (the under-clamp) was mounted on theframe, below the first clamp (upper-clamp) and the heating zone. Withthis under-clamp closed, the length of the piece of fiber in the deviceis fixed and does not change during the treatment. Further, a facilityto cool down the nitrogen flow to temperatures below room temperaturewas introduced.

According to the prior art methods a specific after-treatment can becarried out as follows. For instance, as-spun PIPD fiber, conditioned at21° C. and a relative humidity of 65%, was clamped into the device asdescribed above. Initially, no tension was applied. Then, the tensionwas applied and subsequently the fiber was subjected to one, butpreferably more treatments at different temperatures. The best resultswere achieved with a tension of 300 mN/tex and three periods of heatingof 30 sec, at 150° C., 350° C., and 550° C., respectively. For theevaluation of the mechanical properties, only the part of the fiber wasused that was in the heated area of the oven.

SUMMARY

It is now found that a substantial increase of tensile strength, up to afactor 2 or even more, and an increase of the modulus may be obtained byusing a novel process for obtaining a synthetic organic aromaticheterocyclic rod fiber or film with high tensile strength and/or moduluscomprising spinning a synthetic organic polymer to an aromaticheterocyclic rod fiber or obtaining the synthetic organic polymer as anaromatic heterocyclic rod film (for instance by molding or by using adoctor's blade), followed by loading the fiber or film in the presenceof a processing aid, at a temperature below the boiling point of theprocessing aid and above −50° C., at a tension of 10 to 95% of the fiberor film breaking strength, followed by removing the processing aidand/or performing a heating step at a tension of 10 to 95% of the fiberor film breaking strength.

According to one embodiment of the invention initially no tension wasapplied. Then subsequently, the fiber can optionally be cooled down,preferably at room temperature, and more preferably lower than 20° C.,for instance to 5° C., a tension was applied to the fiber or film (forinstance, about 800 mN/tex) and this tension and temperature weremaintained for a short period, usually less than 1 min, for example for6 sec. Thereafter, the under-clamp was closed i.e. the strain(elongation) of the fiber or film was fixed and heat treatment wasstarted. In this particular case the temperature was raised, forinstance from 5° C. to 500° C. in 1 to 600 sec, or preferably at roomtemperature to 350° C. in 10 to 300 sec.

The mechanical properties of the fibers measured are filamentproperties. They are determined for 25 to 75 filaments by means of aFavimat™ (Textechno, Mönchengladbach, Germany). The average values ofthe breaking tension and the modulus of the filaments were found to be3600 mN/tex and 320 GPa, respectively, measured as the average of 25-75measurements on 25-75 filaments or on 25-75 parts of one or morefilament. The original strength and modulus of the filaments was 2100mN/tex and 170 GPa respectively. For films the measurements were donesimilarly as is known the skilled person.

DETAILED DESCRIPTION OF EMBODIMENTS

In a preferred embodiment the process for making a fiber or film isfurther improved when the spun fiber is subjected to a treatment stepwith the processing aid in the gas or vapor phase at a temperaturebetween 50° and 300° C., preferably between 80° and 100° C., between theloading and heating step, at a tension of 10-95% of the fiber or filmbreaking strength. This treatment with the processing aid in the gas orvapor phase enables the use of lower tension at the subsequent steps,thus leading to less breakage and less fluffs. Particularly, the loadingstep is then performed at lower tension with the same result of highertension loading without applying the treatment with the processing aidin the gas or vapor phase, or at the same tension with higher tenacityand/or modulus than without applying the treatment with the processingaid in the gas or vapor phase. The treatment step with the processingaid in the gas or vapor phase and the heating step can be performed as acombined step wherein the fiber or film is first treated with theprocessing aid in the gas or vapor phase, followed by heating the fiberor film.

The method of the invention can be used for any aromatic heterocyclicrod fibers and films, more preferably PBO and PIPD. The linear densityof the filaments is preferably 0.1 to 5000 dtex, for multifilamentspreferably 0.5 to 5 dtex, more preferably 0.8 to 2 dtex.

The fibers contain one (monofilament) or at least two filaments(multifilament), specifically 2 to 5000, and more specifically 100 to2000. Fibers with about 1000 filaments are commonly used.

The processing aid may be any inert liquid, such as water, acid (e.g.phosphoric acid, sulfuric acid), base (e.g. ammonia), aqueous saltsolutions (e.g. sodium chloride, sodium sulfate), and organic compounds(e.g., ethanediol, methanol, ethanol, NMP). The processing aid ispreferably an aqueous solution, and with more preference water. When theprocessing aid is water, the processing aid in the gas or vapor phase issteam.

According to one embodiment, as-spun fiber or as-obtained film, nothaving received any substantial thermal mechanical after-treatment, ispreferably used. When the fiber is produced by wet spinning or the filmby molding, doctor's blade, or the like, and water or an aqueoussolution is used as the coagulation medium and/or water or an aqueoussolution is used for neutralization and washing, the as-spun fiber oras-obtained film may contain up to more than 100 wt. % of water andafter conditioning at 21° C. and a relative humidity of 65%, the watercontent of the as-spun fiber or as-obtained film may be more than 5 wt.%, typically more than 8 wt. %. In the case of PIPD the moisture contentof the as-spun fiber or as-obtained film after conditioning is about20-24 wt. % (based on dry polymer).

The tension applied during loading and the optional treatment with theprocessing aid in the gas or vapor phase is 10 to 95% of the breakingstrength of the fiber or film, which is higher than the conventionallyused tensions. For instance, in a conventional spinning process of PIPDfibers the loading before drying does not exceed 5% of the breakingstrength of 2100 mN/tex. More preferably, the tension is at least 15%and not more than 80%, most preferably 25 to 60% of the breakingstrength of the as-spun fiber. For film, similar tensions are used. Ifthe treatment with the processing aid in the gas or vapor phase (forinstance a steam treatment) is used the tension during this treatment ispreferably 60-90% of the tension as used during the loading step.Preferably, the treatment with the processing aid in the gas or vaporphase is performed at constant length. Treatment times are between 0.1sec and 1 h, preferably from 1 sec to 300 sec.

The temperature upon loading is below the boiling point of theprocessing aid and at least −50° C., preferably at least −18° C., andmay be near or just above the temperature at which the local thermaltransition of the fiber or film starts as determined with DMTA. Apractical temperature is room temperature. Preferred temperatures arewithin the range between 0° C. and 20° C. For PIPD the local transitiontemperature starts at about −50° C. Typical loading times before heatingare 0.1 sec to 1000 sec.

The heating step includes a temperature above the boiling point of theprocessing aid and may proceed at one temperature or in stages atdifferent temperatures, at atmospheric pressure, at elevated pressure,or, at reduced pressure to promote the removal of the processing aidfrom the fiber or film. The heating step is preferably performed at atemperature of 100° C. up to 50° C. below the melting or decompositiontemperature of the fiber, e.g. in the case of PIPD and PBO 120° C. to450° C., more preferably 125° C. to 350° C., and most preferably, 130°C. to 250° C. for a time between 0.1 sec to 1 h, preferably 1 sec to 300sec. To prevent breaking of the fiber or film at high temperatures, itmay be necessary to decrease the loading gradually during the heatingstep. In a preferred embodiment the processing aid is removedsimultaneously with performing the heating step.

The invention further pertains to a synthetic organic PIPD fiber with alinear filament density between 0.1 and 500 dtex and a tensile strengthhigher than about 3200 mN/tex.

Preferably the tensile strength is higher than 3300 mN/tex, morepreferably higher than 3500 mN/tex. The invention also pertains to asynthetic organic film wherein the modulus of the film is at least 14GPa, preferably at least 20 GPa.

Favimat measurements were performed as follows:

25-75 filaments were randomly selected from a piece of 100 mm of a fiberand suspended in the fiber magazine of a Favimat (Textechno,Mönchengladbach, Germany) with pre-tension weights of 50 mg. From eachfilament the fineness and its force-elongation curve were determinedautomatically, using the following test conditions: temperature 21° 23C. relative humidity 65% gauge length 25.4 23 mm fiber count pre-tension1.0 cN/tex clamp speed 2.54 mm/min

As values for the mechanical properties, the average values of theproperties of the filaments were taken. The following results wereobtained: drying steps 30 sec at 30 sec at 30 sec at loading step (6sec) 150° C. 350° C. 550° C. temperature tension tension tension tensiontenacity elongation modulus entry (° C.) (mN/tex) (mN/tex) (mN/tex)(mN/tex) (mN/tex) (%) GPa prior art no treatment 300 300 300 2556 1.5289 1 5 800 fixed length 3650 1.60 322 2 20 750 fixed length 3118 1.77316 3 −40 750 fixed length 3415 1.97 300 4 5 750 fixed length, heatedfrom 3447 1.88 310 5-500° C. in 600 sec as-spun no treatment 2075 2.83167

1. A process for obtaining a synthetic organic aromatic heterocyclic rodfiber or film with high tensile strength and/or modulus comprisingspinning a synthetic organic polymer to an aromatic heterocyclic rodfiber or obtaining the synthetic organic polymer as an aromaticheterocyclic rod film, followed by loading the fiber or film in thepresence of a processing aid, at a temperature below the boiling pointof the processing aid and above −50° C., at a tension of 10 to 95% ofthe fiber or film breaking strength, followed by removing the processingaid and/or performing a heating step at a tension of 10 to 95% of thefiber or film breaking strength.
 2. The process according to claim 1,wherein the as-spun fiber or the as-obtained film is subjected to theloading step.
 3. The process according to claim 1, wherein the loadingstep is performed between −18° C. and room temperature.
 4. The processaccording to claim 1, wherein the heating step is performed at 100° C.or higher.
 5. The process according to claim 1, wherein the as-spunfiber or the as-obtained film is subjected to a treatment step with theprocessing aid in the gas or vapor phase at a temperature between 50°and 300° C., between the loading step and the heating step.
 6. Theprocess according to claim 1, wherein the processing aid is an aqueoussolution.
 7. The process according to claim 1, wherein the processingaid is removed simultaneously with performing the heating step.
 8. Theprocess according to claim 1, wherein the synthetic organic heterocyclicrod fiber or film is a PIPD fiber or film.
 9. A synthetic organic fiberobtainable by the process of claim 1, wherein the fiber is PIPD with alinear filament density between 0.1 and 500 dtex and an average tensilestrength higher than 3200 mN/tex.
 10. The synthetic organic fiber ofclaim 9, wherein the average tensile strength is higher than 3500mN/tex.
 11. A synthetic organic film obtainable by the process of claim1, wherein the modulus of the film is at least 14 GPa.
 12. The processaccording to claim 3, wherein the loading step is performed between 0and 20° C.
 13. The process according to claim 5, wherein the as-spunfiber or the as-obtained film is subjected to a treatment step with theprocessing aid in the gas or vapor phase at a temperature between 80°and 100° C., between the loading step and the heating step.
 14. Theprocess according to claim 6, wherein the processing aid is water. 15.The synthetic organic film according to claim 11, wherein the modulus ofthe film is at least 20 GPa.